Avast Wi-Fi Inspector is a tool of a different kind. According to the page you referred to, Wi-Fi Inspector scans for vulnerabilities in the network hardware and software to ensure those vulnerabilities don't get exploited and they don't become a botnet.

Our WiFi Guard on the other hand is designed to alert you if there are new and unknown devices in your home network, which would be the case if your WiFi password was compromised or, for example, your neighbours are stealing your Internet. Plus it lets you see which devices have used your network over time. It has a nice user manual that describes in more detail what WiFi Guard can do for you, you may like to have a look at it.